LiTiZn - Ferrite Radome for Satellite Communication

Transcription

1 LiTiZn - Ferrite Radome for Satellite Communication Naveen Kumar Saxena 1 (IEEE Student Member), Nitendar Kumar 2 and P.K.S. Pourush 1 1 Microwave Lab, Department of Physics, Agra College Agra PIN 2822 (U.P) India. Nav391@rediffmail.com, ppourush@yahoo.co.in 2 Solid State Physics Laboratory, Timarpur, Delhi 117 India. Nitendar@rediffmail.com Abstract: The magnetically switchable LiTiZn-Ferrite Radome s dispersion characteristics is presented for the satellite communication. A thin layer of LiTiZn-ferrite is used as superstrate or radome layer which control the radiation, reception, and scattering from a printed antenna or array by applying a dc magnetic bias field in the plane of the ferrite, orthogonal to the RF magnetic field. In this analysis absorbing and transmission power coefficient is calculated to obtain the power loss in radome layer and transmitted power through respectively. The absorbing power coefficient verifies the switching behavior of radome for certain range of applied external magnetic field (H o ) which depends on the resonance width parameter (ΔH) of ferrite material. By properly choosing the bias field, quasi TEM wave propagation in the ferrite layer can be made to be zero or negative over a certain frequency range, results a switching behavior in the ferrite layer. [Nature and Science 29;7(11):9-14]. (ISSN: ). Key words: Substituted Li-ferrite superstrate layer, absorbing and transmission power coefficient, quasi-tem and magnetostatic wave. List of Symbols f r = resonant frequency δ = thickness of radome layer α = attenuation constant β = phase constant β o = propagation constant in vacuum ε r = dielectric constant µ eff = effective permeability µ, k = permeability tensor components of µ eff K d = ordinary propagation constant K e = extraordinary propagation constant T = relaxation time H o = applied bias field ΔH = magnetic resonance width of ferrite ω = angular frequency of incident e-m-waves ω o = external magnetic field angular frequency ω m = internal magnetic field angular frequency µ = real part of permeability µ = dissipative part of permeability χ = real part of susceptibility χ = dissipative part of susceptibility 4πM s = saturation magnetization = gyromagnetic ratio (2.8 MHz / Oe.) 1. Introduction Ferrite materials have a permeability tensor, whose elements can be controlled by the direction and strength of a dc magnetic bias field. A certain frequency range, results an evanescent wave behavior in the ferrite layer, and a large attenuation of the wave transmitted through the layer due to the generation of quasi-tem modes and higher-order modes of the magnetostatic surface wave mode which propagates transversely to the quasi- TEM mode. This reciprocal behavior include the ability to tune the operating frequency of a microstrip antenna, the generation of circular polarization with a single feed point, the dynamic wide angle impedance matching of a phased array, and the reduction of microstrip antenna RCS using a normally biased ferrite substrate. In this work we describe the dispersion characteristics of radome layer by evaluating the absorbing and transmission power coefficients (Pozar et al., 1993, 1992, 1988; Fukusako et al., 1998). With the help of proposed analysis we can also conclude, how a ferrite radome or superstrate layer can be used in conjunction with a printed antenna as a bulk effect switch, whereby the antenna can be turned on or off by applying an appropriate magnetic bias field. This effect makes use of the negative permeability state of an extraordinary quasi TEM plane wave, propagating in a ferrite region. Applications include radar cross section reduction, EMP protection, and possibly a switchable polarizer. The idea of using the negative permeability effect of a ferrite for switching radome is not a new one (Dixit et al., 2; Batchelor et al., 1997; Ufimtsev et al., 2; Horsfield et al., 2); but here a different approach is applying with new ferrite material LiTiZn-ferrite which is synthesized by Solid State Reaction Technique at Solid State Physics Laboratory, Timarpur, Delhi. 9

2 2. Synthesis of Radome Layer LiTiZn ferrite synthesized from the basic components of lithium ferrites In this work a typical composition of LiTiZn ferrite having room temperature magnetization (4πMs) of 22 gauss (± 5%) & Curie temperature (Tc) of 5 C (± 5%) & synthesized using solid state reaction technique (SSRT). The ingredients required for the preparation of these ferrites were calculated on the basis of chemical formula. A small amount of Mn 3+ ion was also incorporated in the basic composition in order to suppress the formation of Fe 2+ ions in the ferrites and to influence megnetostriction being a John Teller ion (Uitert et al., 1956; Kishan et al, 1985). In order to avoid Lithia at high temperatures of sintering, Bi 2 O 3 (.25 wt %) was added as sintering aid (Randhawa et al, 27). Analytical grade chemicals were used for the preparation of the material. The stoichiometric ratio of the chemicals was thoroughly mixed in a polypropylene jar containing the zirconium balls & distilled water was used as a mixing agent. The presintering of the mixed powder has been carried out at ~ 75 o C in a box furnace and soaking time was kept 4 hours. The sieved material was pressed in disk (antenna substrate) and toroidal shapes with the help of suitable dies and using hydraulic pressing technique at pressure of 1 ton/cm 2. The substrates and toroidals were finally sintered at 15 o C for four hours. The heating and cooling cycle of the samples was carried out in the air atmosphere of furnace. The sintered sample so obtained was subjected to cutting, grinding, polishing etc. in order to get specific size and shape. The important material properties such as magnetic and electrical properties were studied. Table1. The electrical and magnetic properties of LiTiZn ferrite substrate LiTiZn Ferrite Characteristics Values Magnetic Saturation ( ) 22 Gauss Curie Temperature (T c ) 385 K Density (ρ) 4.21 grams/cm 3 Remanence.9 Coercivity 1.5 Dielectric Constant (ε) 16 Resonance Line Width ( H) 37 Oersteds Loss Tangent ( ) <.5 The single-phase spinel nature of the samples was confirmed by X-ray diffraction (XRD) patterns obtained by using Cu-K α radiation. The microstructure studies of the sample were carried out by scanning electron microscopy (SEM). Vibrating Sample Magnetometer (VSM) was used to determine the magnetic properties of the samples. For dielectric measurements, rectangular pellets of size 15mm 6mm 3mm were used. The dielectric measurements were conducted from 1 to 2 MHz. by a HP 4192 A impedance analyzer. The value of the real part of dielectric constant ( ) of the ferrite samples was calculated using formula, (where is the permittivity of free space = F/m, C is the capacitance of specimen, t is the thickness of specimen in square meter). The density measurement has been done by a small experiment based on Archimedes' principle. Remanence and Coercive Force measure by B-H loop setup applied to coiled toroid sample at 5 Hz. The Curie temperature for the LiTiZn ferrite samples has been determined by using a simple experimental setup based on gravity effect in the laboratory. The ferrite specimen is made to attach itself to a bar magnet through a mild steel rod due to the magnetic attraction and combination is suspended inside the furnace. A chromel-alumel thermocouple is attached with the sample holder to read the temperature of the specimen. As the temperature of the system is increased, at a particular temperature the specimen losses it spontaneous magnetization and become paramagnetic. This temperature is known as Curie temperature. At this temperature specimen fall downward due to gravity. The electrical and magnetic properties of LiTiZn ferrite substrate is experimentally calculated are presented in table Theory of Operation Consider a plane wave propagating in the perpendicular direction of radome layer with a magnetic bias field applied longitudinally. On the basis of magnetic field directions following waves are generated in the radome layer. 3.1 Magnetostatic mode of wave MSW are generated when external magnetic field applied to the perpendicular direction of the magnetic vector of EM waves. MSW are two types (1) Surface MSW (2) Volume MSW. MSW will propagate perpendicularly on both sides to the EM wave s propagation [Lax et at, 1962]. Vol. MSW: Sur. MSW: 1

3 The absorption and transmission coefficients due to the generation of MSW in the ferrite slab are: where is the effective permeability where where, is the bias field, is the saturation magnetization, is the gyromagnetic ratio as In the case of extraordinary wave mode, the propagation constant dependence on the basic parameters is given as and It is seen that, when is negative, the wave is decaying even if the material is lossless. The frequency range of negative is: where with The frequency limits define the approximate range within and around which the ferrite exhibit interesting microwave characteristics. 4. Setup Figure 1 shows the arrangement of an experimental setup for the validation of the switchable ferrite radome effect. 3.2 Quasi TEM mode of wave As discussed in (Lax et al, 1962; Kabos et al, 1994; Sodha et al, 1981), for a biased ferrite slab, a normal incident plane wave may excite two types of waves (ordinary and extraordinary wave). In the case of normal incident magnetic field biasing ordinary wave is same as the plane wave in the dielectric slab. On the other hand, the extraordinary wave is a TE mode polarized parallel to the biasing direction with its phase propagation constant K e. In the case of extraordinary mode, the propagation constant dependence on the basic parameters is given as Figure 1. Setup for the measurement of radome power coefficients. Ferrite is 2 mm thick with 22 Gauss saturation magnetization, 16 dielectric constant and 18 Oe magnetic resonance width. 11

4 A 2 x 2 circular microstrip patch array was fabricated on a RT-duroid substrate, and operated at 1 GHz. A 1-cm foam spacer separated the array face from the ferrite radome. The ferrite layer was 44mm diameter, and was mounted between the poles of a laboratory electromagnet. An X-band waveguide-tocoax adapter was used as a receiving antenna, and was spaced about 5 cm above the ferrite layer. As illustrated in Figure 1, a ferrite superstrate or radome layer can be placed above a microstrip antenna or array (Or any type of antenna, for that matter), and used as a switch. In practice such a ferrite layer could be spaced a small distance above the antenna, or placed directly over the antenna as a superstrate layer. Spacing the ferrite above the antenna may be preferable for ease of biasing, and also to minimize the direct interaction of the ferrite with the antenna elements (Bahl et al, 198; Balanis et al, 1982) Results When the ferrite layer is unbiased, or biased to a state where K e >, the antenna will transmit and receive as normal. When the ferrite is biased to the cutoff state where K e <, however, an incident wave will be transformed to quasi-tem and magnetostatic waves, which largely absorb and attenuate the incident RF waves. From the graph figure 2 we can see, the absorbing power is max between 17 Oe and 185 Oe which is in good agreement of dispersion graph figure 3 plotted for LiTiZn-ferrite radome layer. Dispersion graph depicts the switch off state of radome layer for cutoff frequency (f) around 5 to 5.5 GHz. From the figure 2, we can also observe the transmitted power coefficient variation with varying external DC magnetic field. Absor. Coeff. Trans. Coeff. Power Coefficient DC Magnetic Field (Ho) 25 Figure 2: Comparison of transmission (T) and absorption (P) power coefficient with the varying DC magnetic field (H o ) Cutoff Frequency (f) 1 x 19 Switchability Region Quasi TEM Wave Excitation (in order of 1-1) Magnetostatic Wave Excitation Spin Wave Excitation Wave Propagation Constant (Ke) 14 x 1 8 Figure 3: Dispersion curve (f Vs. K e ) for incident plane wave perpendicular to biased radome layer 12

5 The amount of absorption and attenuation can be increased by operating the ferrite in a bias state to maximize power loss or by increasing the thickness of the ferrite layer (figure 4 and 5). If desired, dielectric matching layers could be placed on either side of the ferrite layer to reduce reflections. Magnetic and Power Transmission Coeff. (T) dielectric losses will have the effect of increasing the amount of attenuation; as compared to the lossless state (although at the point of maximum cutoff the attenuation may actually decrease slightly with the addition of magnetic losses). h= 1.65 mm h= 2. mm DC Magnetic Field (Ho) Figure 4: Transmission (T) power coefficient with the varying DC magnetic field (H o ) oeff. (P/Po) Power Absorption C h= 1.65 mm h= 2. mm DC Magnetic Field (Ho) Figure 5: Absorption (P) power coefficient with the varying DC magnetic field (H o ) CONCLUSION The Dispersion Characteristics of thin layer radome of LiTiZn-ferrite under external DC-magnetic is presented. Resulted absorbing power coefficients graph, verifies the dispersion relation graph obtained by quasi-tem cutoff frequency range. As discussed, this is a very simple approach which ignores reflections at the ferrite-air interfaces, as well as multiple reflections between the ferrite and antenna layers, but it is found to give a reasonable justification of the attenuation through the radome layer. More sophisticated (e.g., fullwave) analysis may be necessary if the ferrite layer is in direct contact with the antenna or array. It is seen that the frequency where maximum attenuation occurs can be tuned by adjusting the bias field. Also note that the attenuation is greater for higher frequencies, primarily because the ferrite layer looks electrically thicker. 13

6 REFERENCES [1] Pozar D.M., A magnetically switchable ferrite radome for printed antennas, IEEE Microwave and Guided Wave Letters 1993; MWL-3(3): [2] Fukusako T., Seki Y. and Mita N., Dispersion Characteristic of Microstrip Line using Ferrite Substrate Magnetized Longitudinally, Electronics Letters 1998; 34(16): [3] Pozar D. M., Sanchez V., Magnetic tuning of a microstrip antenna on a ferrite substrate, Electronic Letters, vol. 24, pp , June 9, [4] Pozar D. M. RCS reduction for a microstrip antenna using a normally biased femte substrate. IEEE Microwave Guided Wave Letters : [5] Dixit L., Pourush P.K.S. Radiation characteristics of switchable ferrite microstrip array antenna. IEE Proc. Microwave and Antennas Propagation 2; 147(2): [6] Batchelor J.C., Langley R.J. Beam Scanning using Microstrip Line on Biased Ferrite. Electronic Letters 1997; 33(8): [7] Ufimtsev P.Y., Ling R.T., and Scholler J.D. Transformation of surface waves in homogenous absorbing layers. IEEE Transaction on Antennas and Propagation 2; 48: [8] Horsfield B. and Ball J. A. R. Surface wave propagation on grounded dielectric slab covered by a high-permittivity material. IEEE Microwave and Guided wave letters 2; 1: [9] Uitert L.G. Van. Mg-Fe 3+ Spinels (Mg ferrites) and Mg-Fe 3+ Spinels with Substitutions. Proc IRE 1956; 44:1294. [1] Kishan Pran, Sagar D.R., Chatterjee S.N., Nagpaul L.K., Kumar N., Laroia K.K. Optimization of Bi 2 O 3 Contents and its role in Sintering of Lithium Ferrite. Adv in Ceramics 1985; 16:27. [11] Randhawa B.S., Dosanjh H.S., Kumar Nitendar. Synthesis of Lithium Ferrite by Precursor and Combustion Methods: A Comparative Study. Journal of Radio Analytical and Nuclear Chemistry 27; 274(3): [12] Lax B. and Button K. Microwave Ferrite and Ferrimagnetics. McGraw-Hill, New York: [13] Kabos P. and Stalmachov V. S. Magnetostatic Waves and their Applications. Chapman and Hall: [14] Sodha M.S. and Srivastav N.C. Microwave Propagation in Ferrimagnetics. Plenum Press, New York: [15] Bahl I.J. and Bhartia P. Microstrip Antennas. Artech House, Norwood, M.A: 198. [16] Balanis C.A. Antenna Theory Analysis and Design. Harper & Row Publisher, New York (U.S.A.): AUTHORS` BIODATA Naveen Kumar Saxena received his Master degree in Physics (Specialization in Electronics and Communication) from Dr. B. R. Ambedkar University, Agra (UP) India. Presently he is engaged in research for Ph.D. degree in physics (Microwave Science). His current research interest includes ferrite based microstrip antennas and arrays, microwave ferrite materials and artificial neural network analysis. He has attended and presented 3 international conferences and 3 national conferences. Presently he also has student membership of IEEE. Nav391@rediffmail.com Dr. Nitendar Kumar has done B.Sc from Delhi University, & IETE (BE) from Delhi. He obtained his Ph.D. degree in the field of Microwave Ferrimagnetics from Department of Physics, Jamia Millia Islamia, New Delhi in He joined DRDO in year 1981 at Solid State Physics Laboratory (SSPL), Delhi. He has been working in the area of development ferrite and garnet materials for the last 27 years. Presently, he is working in Microwave Group of SSPL as Scientist E. He has been co-recipient of DRDO Technology Award for the year 1995 & 22. He has more than 4 research papers in reputed journals. He was Post doctoral Research Associate for more than one year at Kumamoto University, Japan during 23-4 and worked on ferrite based patch antenna. His main areas of research interests are; ferrite & garnet materials for microwave applications. Nitendar@rediffmail.com Dr. P.K.S Pourush received M.Sc. and M.Phil. degrees in Physics from Agra University in 1986 and 1987 respectively. He has worked on Microstrip Antennas and Arrays at Department of Electrical and Engineering, Malaviya Engineering College Jaipur and received Ph.D. from University of Rajasthan Jaipur in Presently he is working in the Department of Physics, Agra College Agra. His present research interest includes ferrite based microstrip antennas, phased array systems and neural network analysis. He has published around 55 papers in National, International journals and conferences. He also supervised number of M.Phil. and Ph.D. thesis. He has been awarded Young Scientist Award from the institute of Electronics, Information and Communication Engineers FuKuoka Japan during Aug 2. He is also a Fellow of Institution of Electronics and Telecommunication Engineers (IETE) New Delhi and life member of Plasma Science Society of India (PSSI) and Indian Association of Physics Teachers (IAPT). ppourush@yahoo.co.in 1/ 6/29 14